Photon-counting receivers are deployed on the NASA Ice, Cloud and land Elevation Satellite-2 (ICESat2) Advance Topographic Laser Altimeter System (ATLAS). The ATLAS laser altimeter design has total six ground tracks with three strong and three weak tracks. The strong track has nominally 4 times more laser power than the weak track. The receiver is operated in photon counting mode. There are 16 photon-counting channels for each strong track and 4 photon-counting channels for each weak track. Hamamatsu photomultiplier with a 4x4-array anode was used as photon counting detector. This receiver design has high counting efficiency (&gt;15%) at 532 nm, low dark count rate (&lt;400 counts per second), low jitter (less than 285ps), short dead time (&lt;3 ns), long lifetime under large solar background radiation, radiation harden for space operation, and ruggedized for survives the harsh vibration during the launch. In this paper, we will present the initial on-orbit performance of this photon-counting receiver.

Understanding the causes and magnitude of change in the cryosphere remains a priority for earth science research. Over the past decade, NASA earth observing satellites have documented a decrease in both the extent and thickness of Arctic sea ice, and ongoing loss of grounded ice from the Greenland and Antarctic ice sheets. Understanding the pace and mechanisms of these changes requires long-term observations of ice sheets, sea ice thickness and sea ice extent. In response to this need, NASA’s Goddard Space Flight Center (GSFC) is developing the ICESat-2 mission, a nextgeneration laser altimeter designed to measure changes in ice sheet elevation, sea ice thickness, and vegetation canopy height. Scheduled for launch in late 2017 with a three year mission life, ICESat-2 will use a photon-counting micropulse laser altimeter, the advanced topographic laser altimeter system (ATLAS) instrument to collect these key data.

NASA Goddard Space Flight Center (GSFC) has been engaging in Earth and planetary science remote sensing
instruments development for many years. The latest instrument was launched in 2008 to the moon providing the most
detailed topographic map of the lunar surface to-date. NASA GSFC is preparing for several future missions, which for
the first time will perform active spectroscopic measurements from space. In this paper we will review the past, present
and future of space-qualified lasers for remote sensing applications at GSFC.

The Space Infrared Interferometric Telescope (SPIRIT) was designed to accomplish three scientific objectives: (1) learn
how planetary systems form from protostellar disks and how they acquire their inhomogeneous chemical composition;
(2) characterize the family of extrasolar planetary systems by imaging the structure in debris disks to understand how
and where planets of different types form; and (3) learn how high-redshift galaxies formed and merged to form the
present-day population of galaxies. SPIRIT will accomplish these objectives through infrared observations with a two
aperture interferometric instrument. This paper gives an overview into the optical system design, including the design
form, the metrology systems used for control, stray light, and optical testing.

The Wide-Field Imaging Interferometry Testbed (WIIT) was designed to develop techniques for wide-field of view imaging
interferometry, using "double-Fourier" methods. These techniques will be important for a wide range of future space-based
interferometry missions. We have provided simple demonstrations of the methodology already, and continuing
development of the testbed will lead to higher data rates, improved data quality, and refined algorithms for image reconstruction.
At present, the testbed effort includes five lines of development; automation of the testbed, operation in an
improved environment, acquisition of large high-quality datasets, development of image reconstruction algorithms, and
analytical modeling of the testbed. We discuss the progress made towards the first four of these goals; the analytical
modeling is discussed in a separate paper within this conference.

This paper describes computational results obtained with a high-fidelity optical model of the Wide-Field Imaging
Interferometry Testbed (WIIT). The WIIT model includes imperfections inherent in the hardware testbed, such as
deviations of the mirrors from their ideal shapes. Model interferograms (brightness in a detector pixel as a function of
optical delay) are presented here for several representative test scenes "observed" with multiple interferometric
baselines. The results match theoretical expectations and can be compared with real WIIT measurements to identify and
characterize instrumental and environmental artifacts in our laboratory data, and to aid in the interpretation of those data.

We report results of a recently-completed pre-Formulation Phase study of SPIRIT, a candidate NASA Origins Probe mission. SPIRIT is a spatial and spectral interferometer with an operating wavelength range 25 - 400 &#956;m. SPIRIT will provide sub-arcsecond resolution images and spectra with resolution R = 3000 in a 1 arcmin field of view to accomplish three primary scientific objectives: (1) Learn how planetary systems form from protostellar disks, and how they acquire their chemical organization; (2) Characterize the family of extrasolar planetary systems by imaging the structure in debris disks to understand how and where planets form, and why some planets are ice giants and others are rocky; and (3) Learn how high-redshift galaxies formed and merged to form the present-day population of galaxies. Observations with SPIRIT will be complementary to those of the James Webb Space Telescope and the ground-based Atacama Large Millimeter Array. All three observatories could be operational contemporaneously.

The Fourier-Kelvin Stellar Interferometer (FKSI) is a mission concept for a spacecraft-borne nulling
interferometer for high-resolution astronomy and the direct detection of exoplanets and assay of their
environments and atmospheres. FKSI is a high angular resolution system operating in the near to mid-infrared
spectral region and is a scientific and technological pathfinder to the Darwin and Terrestrial Planet
Finder (TPF) missions. The instrument is configured with an optical system consisting, depending on
configuration, of two 0.5 - 1.0 m telescopes on a 12.5 - 20 m boom feeding a symmetric, dual Mach-
Zehnder beam combiner. We report on progress on our nulling testbed including the design of an optical
pathlength null-tracking control system and development of a testing regime for hollow-core fiber
waveguides proposed for use in wavefront cleanup. We also report results of integrated simulation studies
of the planet detection performance of FKSI and results from an in-depth control system and residual
optical pathlength jitter analysis.

We present recent results from the Wide-Field Imaging Interferometry Testbed (WIIT). The data acquired with the WIIT is "double Fourier" data, including both spatial and spectral information within each data cube. We have been working with this data, and starting to develop algorithms, implementations, and techniques for reducing this data. Such algorithms and tools are of great import for a number of future missions, including the Space Infrared Interferometric Telescope (SPIRIT), the Submillimeter Probe of the Evolution of Cosmic Structure (SPECS), and the Terrestrial Planet Finder Interferometer (TPF-I)/Darwin. Recent results are discussed and future study directions are described.

The technique of wide field imaging for optical/IR interferometers for missions like Space Infrared Interferometric (SPIRIT), Submillimeter Probe of the Evolution of Cosmic Structure (SPECS), and the Terrestrial Planet Finder (TPF-I)/DARWIN has been demonstrated through the Wide-field Imaging Interferometry Testbed (WIIT). In this paper, we present an optical model of the WIIT testbed using the commercially available optical modeling and analysis software FRED. Interferometric results for some simple source targets are presented for a model with ideal surfaces and compared with theoretical closed form solutions. Measured surface deformation data of all mirror surfaces in the form of Zernike coefficients are then added to the optical model compared with results of some simple source targets to laboratory test data. We discuss the sources of error and approximations in the current FRED optical model. Future plans to refine the optical model are also be discussed.

The Fourier-Kelvin Stellar Interferometer (FKSI) is a mission concept for an imaging and nulling interferometer in the
near to mid-infrared spectral region (3-8 microns), and will be a scientific and technological pathfinder for upcoming
missions including TPF-I/DARWIN, SPECS, and SPIRIT. At NASA's Goddard Space Flight Center, we have
constructed a symmetric Mach-Zehnder nulling testbed to demonstrate techniques and algorithms that can be used to
establish and maintain the 10<sup>4</sup> null depth that will be required for such a mission. Among the challenges inherent in such
a system is the ability to acquire and track the null fringe to the desired depth for timescales on the order of hours in a
laboratory environment. In addition, it is desirable to achieve this stability without using conventional dithering
techniques. We describe recent testbed metrology and control system developments necessary to achieve these goals
and present our preliminary results.

The goal of the Terrestrial Planet Finder (TPF) mission is to detect and characterize terrestrial exoplanets at visible wavelengths. One approach combines an 8m by 3.5m aperture telescope with a coronagraph (TPF-C) to obtain the required planet to parent star contrast. The proposed design places severe constraints on alignment tolerances and requires optics of the highest possible quality. The integration, test and verification of the observatory will require extraordinary procedures. This paper is an initial attempt to outline a plausible program to verify before launch the in-orbit performance requirements.

The Fourier-Kelvin Stellar Interferometer (FKSI) is a mission concept for an imaging and nulling interferometer for the near infrared to mid-infrared spectral region (3-8 microns). FKSI is a scientific and technological pathfinder to TPF/DARWIN as well as SPIRIT, SPECS, and SAFIR. It will also be a high angular resolution system complementary to JWST. There are four key scientific issues the FKSI mission is designed to address. First, we plan to characterize the atmospheres of the known extra-solar giant planets. Second, we will explore the morphology of debris disks to look for resonant structures to find and characterize extrasolar planets. Third, we will observe young stellar systems to understand their evolution and planet forming potential, and study circumstellar material around a variety of stellar types to better understand their evolutionary state. Finally, we plan to measure detailed structures inside active galactic nuclei. We report results of simulation studies of the imaging capabilities of the FKSI with various configurations of two to five telescopes including the effects of thermal noise and local and exozodiacal dust emission. We also report preliminary results from our symmetric Mach-Zehnder nulling testbed.

We present recent results from the Wide-Field Imaging Interferometry
Testbed (WIIT). Using a multi-pixel detector for spatial multiplexing, WIIT has demonstrated the ability to acquire wide-field imaging interferometry data. Specifically, these are "double Fourier"
data that cover a field of view much larger than the subaperture diffraction spot size. This ability is of great import for a number of proposed missions, including the Space Infrared Interferometric Telescope (SPIRIT), the Submillimeter Probe of the Evolution of Cosmic Structure (SPECS), and the Terrestrial Planet Finder (TPF-I)/DARWIN. The recent results are discussed and analyzed, and future study directions are described.

We discuss the procedure used to characterize the Wide-Field Imaging
Interferometry Testbed (WIIT) components and system, including
spectral transmission, throughput, wavefront quality, mechanical and
thermal stability, and susceptibility to turbulence. The sources of
uncertainty and visibility loss are identified and evaluated, and we
briefly discuss measures taken to mitigate these effects. We further
discuss calibration techniques which can be used to compensate for
visibility loss factors, and describe the applicability of these
calibration techniques to the future space-based far-IR interferometry
missions SPIRIT (Space Infrared Interferometric Telescope) and SPECS
(Submillimeter Probe of the Evolution of Cosmic Structure).

The Submillimeter Probe of the Evolution of Cosmic Structure (SPECS) is a space-based imaging and spectral ("double Fourier") interferometer with kilometer maximum baseline lengths for imaging. This NASA "vision mission" will provide spatial resolution in the far-IR and submillimeter spectral range comparable to that of the Hubble Space Telescope, enabling astrophysicists to extend the legacy of current and planned far-IR observatories. The astrophysical information uniquely available with SPECS and its pathfinder mission SPIRIT will be briefly described, but that is more the focus of a companion paper in the Proceedings of the Optical, Infrared, and Millimeter Space Telescopes conference. Here we present an updated design concept for SPECS and for the pathfinder interferometer SPIRIT (Space Infrared Interferometric Telescope) and focus on the engineering and technology requirements for far-IR double Fourier interferometry. We compare the SPECS optical system requirements with those of existing ground-based and other planned space-based interferometers, such as SIM and TPF-I/Darwin.

The Wide-Field Imaging Interferometry Testbed (WIIT) will provide valuable information for the development of space-based interferometers. This laboratory instrument operates at optical wavelengths and provides the ability to test operational
algorithms and techniques for data reduction of interferometric
data. Here we present some details of the system design and
implementation, discuss the overall performance of the system to
date, and present our plans for future development of WIIT. In
order to make best use of the interferometric data obtained with
this system, it is critical to limit uncertainties within the
system and to accurately understand possible sources of error. The
WIIT design addresses these criteria through a number of ancillary
systems. The use of redundant metrology systems is one of the most
important features of WIIT, and provides knowledge of the delay
line position to better than 10 nm. A light power detector is used
to monitor the brightness of our light sources to ensure that small
fluctuations in brightness do not affect overall performance. We
have placed temperature sensors on critical components of the
instrument, and on the optical table, in order to assess environmental effects on the system. The use of these systems provides us with estimates of the overall system uncertainty, and allows an overall characterization of the results to date. These estimates allow us to proceed forward with WIIT, adding rotation stages for 2-D interferometry. In addition, they suggest possible avenues for system improvement. The possibility exists to place WIIT inside an environmentally controlled chamber within the Diffraction Grating Evaluation Facility (DGEF) at Goddard in order to provide maximum control over environmental conditions. Funding for WIIT is provided by NASA Headquarters through the ROSS/SARA Program and by the Goddard Space Flight Center through the IR&amp;D Program.

In order for data products from WIIT to be as robust as possible, the alignment and mechanical positions of source, receiver, and detector components must be controlled and measured with extreme precision and accuracy, and the ambient environment must be monitored to allow environmental effects to be correlated with even small perturbations to fringe data. Relevant detailed anatomy of many testbed components and assemblies are described. The system of displacement measuring interferometers (DMI), optical encoders, optical alignment tools, optical power monitors, and temperature sensors implemented for control and monitoring of the testbed is presented.

The Wide-field Imaging Interferometry Testbed was designed to validate, experiment with, and refine the technique of wide field mosaic imaging for optical/IR interferometers. We offer motivation for WIIT, present the testbed design, and describe algorithms that can be used to reduce the data from a spatial and spectral Michelson interferometer. A conventional single-detector Michelson interferometer operating with narrow bandwidth at center wavelength lc is limited in its field of view to the primary beam of the individual telescope apertures, or ~&lambda;<sub>c</sub>/d<sub>tel</sub> radians, where dtel is the telescope diameter. Such a field is too small for many applications; often one wishes to image extended sources. We are developing and testing techniques analogous to the mosaicing method employed in millimeter and radio astronomy, but applicable to optical/IR Michelson interferometers, in which beam combination is done in the pupil plane. An N<sub>pix</sub> &times; N<sub>pix</sub> array detector placed in the image plane of the interferometer is used to record simultaneously the fringe patterns from many contiguous telescope fields, effectively multiplying the field size by N<sub>pix</sub>/2, where the factor 2 allows for Nyquist sampling. This technique will be especially valuable for interferometric space observatories, such as the Space Infrared Interferometric Telescope and the Submillimeter Probe of the Evolution of Cosmic Structure.

The Solar Terrestrial Relations Observatory (STEREO) is a pair of identical satellites that will orbit the Sun so as to drift ahead of and behind Earth respectively, to give a stereo view of the Sun. STEREO is currently scheduled for launch in November 2005. One of the instrument packages that will be flown on each of the STEREO spacecrafts is the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI), which consists of an extreme ultraviolet imager, two coronagraphs, and two side-viewing heliospheric imagers to observe solar coronal mass ejections all the way from the Sun to Earth. We report here on the inner coronagraph, labeled COR1. COR1 is a classic Lyot internally occulting refractive coronagraph, adapted for the first time to be used in space. The field of view is from 1.3 to 4 solar radii. A linear polarizer is used to suppress scattered light, and to extract the polarized brightness signal from the solar corona. The optical scattering performance of the coronagraph was first modeled using both the ASAP and APART numerical modeling codes, and then tested at the Vacuum Tunnel Facility at the National Center for Atmospheric Research in Boulder, Colorado. In this report, we will focus on the COR1 optical design, the predicted optical performance, and the observed performance in the lab. We will also discuss the mechanical and thermal design, and the cleanliness requirements needed to achieve the optical performance.

A combination of a single mode AlGaAs laser diode and a broadband LED was used in a Michelson interferometer to provide reference signals in a Fourier transform spectrometer, the Composite Infrared Spectrometer, on the Cassini mission to Saturn. The narrowband light from a laser produced continuous fringe throughout the travel of the interferometer, which were used to control the velocity of the scan mechanism and to trigger data sampling. The broadband light from the LED produced a burst of fringes at zero path difference, which was used as a fixed position reference. The system, including the sources, the interferometer, and the detectors, was designed to work both at room temperature and at the instrument operating temperature of 170 Kelvin. One major challenge that was overcome was preservation, from room temperature to 170 K, of alignment sufficient for high modulation of fringes from the broadband source. Another was the shift of the source spectra about 30 nm toward shorter wavelengths upon cooldown.

The Composite Infrared Spectrometer (CIRS) of the Cassini mission to Saturn has two interferometers covering the far infrared and mid infrared wavelength region. The mid infrared wavelength interferometer has a focal plane consisting of a germanium focus lens and HgCdTe array. System level calibration of the CIRS Flight Unit indicated a discrepancy between the expected and actual signal levels. Testing on the CIRS breadboard and Engineering Unit indicated that defocus of the germanium lens could significantly reduce the modulation efficiency of the interferometer in the presence of a moderate degree of wavefront shear. Defocus of the lens in the focal plane was of concern because of the temperature dependence of the index of refraction of germanium and the nominal operation temperature of 170 K. The shear/defocus interaction was extensively investigated and correlated to a newly developed analytical model. It was eventually determined that the CIRS instrument was in focus, had no appreciable wavefront shear and was operating near theoretical limits. The shear/defocus effect is however, of considerable interest, since it has not been described in previous literature on interferometers.

The Composite Infrared Spectrometer of the Cassini mission to Saturn has two interferometers covering the far infrared and mid infrared wavelength region. The instrument was aligned at ambient temperature, but operates at 170 Kelvin and has challenging interferometric alignment tolerances. Cryogenic alignment tests of the instrument indicated that it should suffer minimal degradation due to the cooldown from ambient to operational temperature. System level tests performed by the calibration team indicated a lower than expected signal level on the mid infrared channel, while providing ambiguous optical throughput data. Therefore it became imperative to develop a metric that could be used to determine the instrument performance at both the instrument and system levels, at ambient and cryogenic temperature. Modulation efficiency and throughput measurements were performed and new analytical models developed to evaluate the status of the instrument. Methodologies are detailed, empirical and analytical data are reconciled and deviations from design values explained.

The combined effects on performance of shear between the two arms, defocus of the detector, and difference in wavefront between the two arms of a Fourier transform spectrometer using cube corner retroreflectors were investigated. Performance was characterized by the amplitude of the fringe signals coming from a detector as the path-length difference was scanned. A closed-form expression was found for the combined effects of shear and defocus, and it was found that defocus had no effect in the absence of shear. The effect of wavefront error was modeled numerically and assumed to be independent of shear and defocus. Results were compared with measurements make on the breadboard and engineering model of the Composite Infrared Spectrometer for the Cassini mission to Saturn, and good agreement was found.

The composite infrared spectrometer (CIRS) is a remote sensing instrument to be flown on the Cassini orbiter. CIRS will retrieve vertical profiles of temperature and gas composition for the atmospheres of Titan and Saturn, from deep in their tropospheres to high in their stratospheres. CIRS will also retrieve information on the thermal properties and composition of Saturn's rings and Saturnian satellites. CIRS consists of a pair of Fourier Transform Spectrometers (FTSs) which together cover the spectral range from 10-1400 cm<SUP>-1</SUP> with a spectral resolution up to 0.5 cm<SUP>-1</SUP>. The two interferometers share a 50 cm beryllium Cassegrain telescope. The far-infrared FTS is a polarizing interferometer covering the 10-600 cm<SUP>-1</SUP> range with a pair of thermopile detectors, and a 3.9 mrad field of view. The mid-infrared FTS is a conventional Michelson interferometer covering 200-1400 cm<SUP>-1</SUP> in two spectral bandpasses: 600-1100 cm<SUP>- 1100-1400 cm(superscript -1</SUP> with a 1 by 10 photovoltaic HgCdTe array. Each pixel of the arrays has an approximate 0.3 mrad field of view. The HgCdTe arrays are cooled to approximately 80K with a passive radiative cooler.

Optical digital receivers are being considered for intersatellite laser communication links. A demonstration system is being designed to operate at 325 MBit/s, using quaternary pulse position modulation (QPPM). Laboratory experiments have been conducted using a 50 MBit/s prototype system. A numerical model has been developed to simulate a QPPM optical receiver. The 50 MBit/s system was simulated to verify the validity of the model. The model was then used to simulate the projected behavior of the 325 MBit/s system. The model predicts a bit error rate of 10.6 at 38 incident photons per bit for 820 nm light.

A proof-of-concept (POC) demonstration system has been developed which demonstrates acquisition, tracking and point-ahead angle sensing for a space optical communications terminal utilizing a single high speed area array detector. The detector is the 128 x 128 pixel Kodak HS-40 photodiode array. It has 64 parallel readout channels and can operate at frames rates up to 40,000 frames/sec with rms readout noise of 20 photoelectrons. A windowing scheme and special purpose digital signal processing electronics are employed to implement acquisition and tracking algorithms. The system operates at greater than 1 kHz sample (frame) rates. Acquisition can be performed in as little as 30 milliseconds with less than 1 picowatt of 0.85 micron beacon power on the detector. At the same power level, the rms tracking accuracy is approximately 1/16 pixel. Results of system analysis and measurements using the POC system are presented.

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Advanced PhotonicsJournal of Applied Remote SensingJournal of Astronomical Telescopes Instruments and SystemsJournal of Biomedical OpticsJournal of Electronic ImagingJournal of Medical ImagingJournal of Micro/Nanolithography, MEMS, and MOEMSJournal of NanophotonicsJournal of Photonics for EnergyNeurophotonicsOptical EngineeringSPIE Reviews